Electrical current flow through conductors in multiple layered, printed circuit boards (PCBs) often contributes to crosstalk between and among conductors thus limiting the effectiveness and reliability of an application. Conductors in the form of clad runs are mutually influenced depending on their proximity to each other as well as signal power levels, switching rise and fall times and frequency rates. For many radio frequency (RF) applications the degree of isolation between and among conductors and other circuit features such as vias needs to exceed 65 dB. Crosstalk generally occurs by: (1) radiation from one conductor through the substrate and received by another; and/or (2) radiation outside the substrate from one electrical conductor and received by another on the substrate. To reduce crosstalk between clad runs metal shields are often employed. Another method for isolating an electrical feature within a PCB dielectric substrate is to surround the feature with metalized stitching vias connected to ground at top and bottom. These vias may take on one of several different patterns, such as uniform rows, double rows and randomized positioning.
A PCB is a composite dielectric material of organic and/or inorganic fabric impregnated with resin having a copper-clad laminate bonded thereto. The copper is then patterned using resists and etched. A multilayer PCB is achieved when two or more sheets of dielectric material having patterned copper bonded thereto are laminated under heat and pressure. The interconnection between components and conductors in these dielectric layers are effectuated through via holes, which are metal plated to provide the electrical connections between respective layers. Ground and power planes distribute voltages to the circuit components. A prior art configuration typical of a stripline design is shown in FIG. 1. Note in FIG. 1 that the PCB 100 has a dielectric layer 102 and an internal conductor (stripline) embedded therein at a location indicated by dashed line 120a that is circumscribed by a stitching via scheme 105 of uniform double rows of metalized vias, although other designs may use single and randomized via positions. These stitching vias serve to electrically isolate the conductor at line 120a from the adjacent conductor embedded in PCB 100 at a location indicated by dashed line 120b. The isolation provided by any of these methods is limited to the how close one feature, such as a via 125, can be installed to another feature such as a connector installed in slot 107. The effectiveness of the stitching via method of isolation diminishes as the operating frequency increases.
The problem of mutual coupling also arises in substrates other than PCBs. By way of example, the problem of mutual coupling may be encountered in thick film technology.